For increasingly complex electronic and computer devices, power source devices become increasingly important. Power source devices can be divided into two categories: linear type and switching type. Because linear-type power source devices have more disadvantages, all power source devices manufactured now all of the switching-type.
The power supply for a backlight source of a TFT LCD panel makes use of an inverter circuit to convert DC power into AC power for energy conversion and driving of a cold cathode fluorescent lamp (CCFL). Conventional inverter circuits can be divided into half-bridge-type, full-bridge-type and push-pull-type according to different circuit topologies. An inverter circuit is a circuit for converting DC power into AC power.
As shown in FIG. 1, a transformer T2 divides the circuit into a front-end circuit at the primary side 201 and a rear-end circuit at the secondary side 202.
The front-end circuit at the primary side 201 comprises a DC voltage source Vcc, two electronic switches (Q1, Q2), a half-bridge-type control chip TL494, two capacitors (C1, C2) and an isolation transformer Tr. The rear-end circuit at the secondary side 202 comprises a load. Reference is also made to FIG. 2. The half-bridge control chip TL494 outputs control signals D1–D2 via two output terminals D1 and D2. The control signals D1–D2 control switching actions of the two electronic switches Q1 and Q2 via the isolation transformer Tr, respectively. The two electronic switches Q1 and Q2 are n-channel FETs or p-channel FETs. Through switching actions of the two electronic switches Q1 and Q2, electric energy stored in the capacitors C1 and C2 can be transferred to a primary side terminal T21 of the transformer T2 via a coupling capacitor C3 to form AC power source ac. The voltage of the capacitors C1 and C2 is half (Vcc/2) that of the DC voltage Vcc. The AC power source ac is used to provide energy for the transformer T2, which boosts and converts the ac power source to the secondary side 202 for driving the load.
The above conventional half-bridge-type inverter circuit requires an isolation transformer to control switching actions of the electronic switches Q1 and Q2. The half-bridge-type control chip TL494 can't directly drive the electronic switches Q1 and Q2. Moreover, both the electronic switches Q1 and Q2 used in the conventional half-bridge-type inverter circuit are n-channel FETs or p-channel FETs to cause larger energy loss.
Reference is made to FIG. 1 again. The actions of the first switch Q1 and the second switch Q2 are staggered by a small period of time, and are on/off alternately. Therefore, a small period of dead time is required for the control signals D1–D2 shown in FIG. 2 to avoid burnout of the transformer T2 due to the first and second switches Q1 and Q2 being simultaneously on. Moreover, the control signals D1–D2 need to control and change the switching actions of the first and second switches Q1 and Q2 for providing the required power for the load based on the power requirement of the load. Additionally, the control signals D1–D2 need to stop the switching actions of the first and second switches Q1 and Q2 for accomplishing circuit protection according to various situations at the load end and various working statuses of the circuit, such as a too-low output voltage, a too-high output voltage, an open-circuited lamp or a too-high temperature.